The present invention relates generally to material processing and optical apparatus techniques. More particularly, embodiments of the invention provide methods and systems for forming a layer of material that can be used in an optical device, such as a phosphor layer for a lens in an LED device.
Phosphor materials have been widely used in LED package for producing white light, or various colors of light (e.g. phosphor-converted green or red) with blue pump LEDs. There are several methods of depositing a phosphor material over a blue LED chip or package assembly today. As used herein, “phosphor” refers to any luminescent materials, which absorb light of one wavelength and emit light of a different wavelength. As known, there have several advantages to form phosphor component separately on encapsulation such as lens or cover before deposit or attach over LED chips. For example, increasing optical stability for long lifetime of devices as described in U.S. Pat. No. 6,204,523, or improving efficiency of devices by positioning the phosphor layer away from the LED as described in U.S. Pat. No. 6,812,500 and U.S. Pat. No. 7,479,662.
In addition, the separate phosphor component can be pre-optimized for optical performance such as light uniformity, as well as pre-characterized and binned to match LEDs of specific wavelength for improving consistency of color temperature, color quality, and even color yields in manufacturing.
Slurry methods or electrophoretic deposition (EPD) are some of the common methods of forming phosphor particle layer on LED chip or lens surface in manufacturing today. Chromaticity of phosphor converted LED is highly dependent on the properties of the phosphor particle layer, such as packing density, thickness, and uniformity on surface.
In slurry methods, phosphor particles are dispersed into silicone, epoxy or solvent filler material to form a phosphor mixture, applying said phosphor mixture to LED surface or package lens material by various techniques such as spray coating, or dipping coating, or dispensing or phosphor-in-cup or over molding on a support structure and so on. For slurry methods, common or known difficulties in applying phosphor layer to LED chip or package are many, such as                Variation of thickness uniformity on surface. The slurry method usually forms a particle layer with varying thickness, especially applied on non-flat surface.        Difficulties in achieving high packing density of thin particles layers. Therefore, the coated layer contains a high percentage of binder materials, which is typically not a good thermal conductor, increasing the difficulty in dissipating heats for the package.        
In electrophoretic deposition (EPD), although a high packing density of phosphor particles can be formed with a thickness less than 20 um, the known difficulties in EPD technique are                Weak adhesion strength of phosphor particles increases handing difficulty during intermediate steps of fabrication.        Difficult to achieve highly uniform layer across larger area of surface.        Process requires a conductive surface. Extra chemical steps will be required if applied for non-conductive surface, and therefore increases process complexity.        
In practice, therefore, using existing techniques of LED phosphor deposition described previously for pre-coating encapsulation material, which is usually an insulator, and has a non-flat surface, such as lens, with a high packing density of uniform phosphor particle layer is very challenge for applications in phosphor-converted LED today.
Accordingly, there is a need for improved coating techniques.